Mr. Monk and the Panic Room - S3-E2
Factual error: Disher hands Stottlemeyer a pistol and tells him that he just unloaded it, but Stottlemeyer just takes his word for it instead of double-checking it. You NEVER, under any circumstance, take anyone's word that a gun is unloaded; you always check it yourself. As a police officer, Stottlemeyer would know that.